Heat treatment of aluminummagnesium alloys



Patented F ch. 6, 1934 HEAT TREATIWENT OF ALUMINUM MAGNESIUM ALLOYS Edgar H. Dix, Jr., Oakmont, Pa., and Louis W. Kempf, Cleveland, Ohio, assignors to Aluminum Company of America, Pittsburgh, Pa a corporation of Pennsylvania 1 No Drawing. Application December 23, 193% Serial No. 5%,278

6 Claims. (GE. 148-211) This invention relates to aluminum base alloys containing substantial amounts of magnesiurn, and one of its objects is to improve the physical properties of this class of alloys by suit- 5 abie heat treatments. Another object is to regulate or alter by heat treatment certain of the physical properties of articles made from these iioys to produce various commercially useful conditions in the internal structure of the arti- 19 cies.

The recent successful developments in the procluetion of aluminum base alloys containing substantial amounts of magnesium have now made pcss ie the production of clean sound castings 'num base alloys with magnesium within where the greatest difficulty was forultered, namely, from about 5 per cent G per cent of magnesium.

it a very desirable constituent in aluminum a1- loys but because of the difiiculties heretofore encountered in making clean and sound castings of such alloys containing more than about 5 per cent magnesium, alloys of this type have not been used to any extent except in rare instances where the ordinary standards of tensile strength, ductility and. soundness were made subservient to some other property usually not seriously considered in appraising the usefulness of an alloy. Simultaneously with the development of casting methods as noted above, we have been investigating the possibility of improvement or alteration of the properties and internal structure of these alloys by causing the soluble constituent or constituents of the alloy to go into solid solution, and the methods of heat-treatment by which such properties and qualities are produced are embraced within the subject matter of the present application. We have found that the herein described alloys are susceptible to improvement by heat treatment both in the as-cast and in the worked condition, the working being ef- ,fected by any of the known methods, such as rolling, forging, extrusion, etc.

In practicing one of the steps of our invention we take an aluminum base alloy casting containing from about 5 per cent to 20 per cent magnesium and perhaps smaller quantities of other alloying elements and heat it for a period of time at a temperature such as hereinafter discussed.

We have found it necessary to keep the casting at heat at least 5hours to produce advantageous eiiects and for the best results a considerable increase in time is desirable. As a standard heat treating time we use 20 hours although times even longer than this have been found of advantage in the case of the higher amounts of magnesium. Articles which have been rolled, forged, extruded, or otherwise subjected to working processes will respond to heat treatment more rapidly than castings although they may, with good effect, be given the same treatment as that outlined above.

As indicating the beneficial effect of extended time at heat, we give below in Table A typical results with sand cast tensile specimens containing 14 per cent magnesium and heat treated at 445 C. for the times stated in the table. The specimens were cooled by air blast:

. A Table Tensile strength Elongation lbs. per in 2 sq. in.

Hours at 445 0.

With lower amounts of magnesium the heat treating temperature may be somewhat reduced so and the time somewhat shortened. The following Table B shows the effect of heat treating sand cast tensile specimens of 10 per cent magnesium, balance aluminum, at 427 C., for the As described in a copending application of L. W. Kempf, Serial No. 454,523, a material improvement in properties accompanies the use of high purity ingredients for compounding the mo 0.3 per cent impurities.

Table 0 t tt El a 5 ten 01188 on Hours tit-125 0. nm in With extended time a marked increase in both tensile strength and elongation is noted, and for this reason when heat treating quantities of miscellaneous castings we prefer to use at least an hours, even extending this time if optimum results are needed.

We prefer to restrict our heat treating range to lower than 451 0., as we have observed that at this temperature the magnesium constituent which has not gone into solution forms a low melting point aggregate with the aluminum matrix. Temperatures in the neighborhood of 380 to 425 C. have been found to be of fairly general usefulness, and for the best results articles should be heated as high as convenient without too closely approaching 451 C. Higher temperatures have the effect of shortening the time in which identical results are obtained, and with lower temperature the time at heat is correspondingly lengthened. Temperatures as low as 250 C. effect an advantageous improvement, especially with alloys of lower magnesium contents, such as those in the neighborhood of 5 per cent. As the amount of magnesium is increased we find it advantageous to raise the temperature, and in ordinary commercial practice lrzes uually employ a standard temperature of With magnesium contents higher than about 10 per cent we have found that if heated articles are quenched in water cracks may form which will render the articles useless or cause considerable decrease in strength and ductility. We have also found that if the specimens be cooled more slowly, as in air, the results are highly satisfactory. Articles may even be slowly cooled in the-furnace over a period of several hours to about 300 C. and then cooled in air without appreciable diminution in properties. This characteristic of the alloy is especially useful in the case of articles which by reason of intricacy of design or variation of section show a tendency to crack it rapidly quenched, and in this respect our method differs notably-from that practiced in the case of other commercial aluminum alloys.

We have found that lesser amounts of other alloying constituents may be included without rendering the alloy insensitive to improvement by heat treatment and, as a matter of fact, sometimes enhancing the improvement obtained. As an instance of the effect of adding 0.5 per cent of zinc to an aluminum base alloy containing 10 per cent magnesium, we have tested specimens of this composition both in the "as-cast condition and after a heat treatment of 20 hours at 425 C. with the results shown in the following Table D: 1

Table D Condition Elo n gg t ion As-cast We have found that the usefulness of these heat treating processes is not restricted to ascast articles but that the processes may also -be directed towards the alteration or improvement of the properties and internal structure of Wrought articles fabricated by such methods as rolling, forging, extrusion, pressing, etc.

We have also found that the alloys within the scope of this invention are susceptible to increase the tensile strength and. hardness by being subjected to heating for extended periods of time at lower temperatures. Table E, immediately below, indicates the efiect on tensile specimens of aging at 154 C. after a high temperature treatment. The tensile specimens were cut from 14 gage sheet made from an aluminum base alloy containing 10 per cent of magnesium.

Table E Tensile strength lbs. per sq. in.

Scleroscops As-quenched 24 hrs. at 154 C 70 hrs. at 154 0.-

In aluminum base alloys containing magnesium in amounts ranging from about 5 per cent to about 20 per cent the heat treating processes described in this specification may be utilized even though there be present one or more other alloying constituents in lesser amounts. Certain alloying elements in addition to magnesium, as for example manganese, chromium, and nickel, are even found to be distinctly helpful in imparting additional improvement eifects to the articles heat treated by the methods herein 615- closed. .It should also be understood that the aluminum ingot used in compounding these alloys may contain an appreciable amount of impurities such as iron, silicon, copper and lesser quantities of other elements and that some of these may combine with a relatively small amount of the magnesium or may form compounds with each other, and may be caused to undergo structural variations by our heat treating process, but although we recognize the presence of these elements we do not regard them as essential to the success of the heat treating processes we have devised, and we may, in certain cases, even desire to eliminate as far as possible these impurities as being detrimental to the best properties 40 in the alloy. As an aid in outlining the limits of the invention, we postulate an alloy of an aluminum base with magnesium varying between the limits 5 and 20 per cent and have devised We claim- 1. The method of treating an aluminum base alloy containing from about 5 to 20 per cent magnesium to improve the physical properties thereof, comprising heating the alloy for about 5 to 20 hours at a temperature from about 250 to 450 C., and cooling the alloy rapidly enough to preserve the benefits of the heating.

2. The method of treating an aluminum base alloy containing from about 10 per cent to 20 per cent magnesium to improve the physical properties thereof, which comprises heating the alloy between about 380 .C. and about 450 C. for about 5 to 20 hours, and cooling the alloy rapidly enough to preserve the benefits oi the heating.

3. The method of treating an aluminum base alloy containing from about 5 to 15 per cent than '20 hours at from about 250' C. to about 450 0., cooling the alloy rapidly enough to retain the benefits of the heat treatment, and thereafter re-heating the alloy at a temperature less than 200 C. for a period of time suiflcient to increase the strength and hardness oi the alloy. I

5. The method'oi treating a casting made of an aluminum base alloy containing from about 5 to 20 per cent magnesium,which comprises heating the casting for more than about 5 but not more than about 20 hours, at from about 250 C. to about 450 C., cooling the casting rapidly enough to retain the benefits of the heat treatment but slowly enough to avoid cracking of the casting, and thereafter re-heating the alloy at a temperature from about 150 C. to 200 C. to increase the strength and hardness of the casting.

6., The method oi. treating a casting made of an aluminum base alloy containing from about 5 to 20per cent magnesium comprising, heating the casting for more than about 5 hours but not -more than about 20 hours at a temperature of from 425 C. to 50 C., cooling the casting slowly to about 300 C., and then cooling the casting in air to room temperature.

\ EDGAR H. DIX, JR.

LOUIS W. KEMPI". 

